1. Field of the Invention
This invention relates to electronic musical instruments, and more particularly to an improved contour generating circuit for use in such instruments.
2. The Prior Art
In electronic musical instruments such as electronic organs and synthesizers, the sound which is obtained when a particular note or combination of notes is played is determined by a number of factors including the frequency of the audio signal, the shape of the signal (i.e., harmonic content thereof) and the contour (i.e., shape of the envelope) to the signal. The contour of the signal determines the amplitude of the signal at each point in time during the period that the note is being sounded. Typically, the amplitude of the signal rises to a predetermined maximum value over a predetermined time interval (attacktime), starting when a key or switch of the instrument is operated, remains at this maximum level, or decreases over a predetermined time interval to a predetermined sustain level, while the key or switch is operated, and then decays over a predetermined time interval when the key is released or the switch opened. The attack and decay characteristics may have any desired shape, a linear or an exponential characteristic being the two most common.
In devices of the type indicated above, the attack and decay slopes have normally been controlled from a current source, a constant current source being required for linear slope characteristics. These current sources have had to be bi-directional in order to permit control of both attack and decay slopes. Further, only one type of slope characteristic (i.e., linear or exponential) has been available with any given contour generator circuit, a completely separate circuit being required if a capability for different slope characteristics is required. The factors indicated above have made it difficult to provide a voltage control for the slope of the contour and have generally caused these circuits to be relatively complicated and expensive.
A need therefore exists for a relatively simple and inexpensive contour generating circuit for an audio signal which permits the slope for both attack and decay to be controlled by currents flowing in the same direction and thus permits the slope to be voltage controlled. Such a circuit should also permit different attack and decay curve characteristics, such as for example linear or exponential characteristics, to be achieved utilizing basically the same circuit.